The Challenge

Fluid bed dryers utilize hot air forced up beneath a bed of moist (yet combustible) dust in the drying process. This ensures that a turbulent dust suspension, and therefore an explosive situation exists in the head space of the dryer at all times.

In addition, in certain applications, high levels of dust are also being carried away by the dust extraction system so an explosion can spread quickly and easily to other components associated with the dryer.

The ignition sources can be the spontaneous combustion of materials that accumulate on the walls, roof or wires of the dryer or an electrostatic discharge due to a failure in the grounding system.

Confining and suppressing the explosion is of paramount importance when designing protection for fluid bed dryers. The explosion protection system must cover the main volume of the dryer as well as all inlets and outlets.

the hazard

If the material being handled by a process is combustible, then the dust it generates is likely to be explosive.  Fluid bed dryers/coolers generate a potentially explosive dust/air mixture as the material being dried or cooled is fluidized by the incoming air.  Under these conditions, an ignition source is all that is required to trigger an explosion.  This can be provided by incoming burning particles from upstream equipment such as other dryers.  Material adhering to the walls of the dryer or material building up at the internal weir can also auto-ignite providing the ignition for an explosion.

protection system description

Fluid bed dryers/coolers are commonly located inside the plant making protection by explosion relief venting impractical.  Their geometry also makes venting difficult to accomplish.  An explosion suppression system provides the solution.  In such a system, explosion pressure detectors mounted on the fluid bed dryer/cooler detect the pressure excursion from an incipient explosion.  The detectors transmit a signal to a control panel, which triggers high rate discharge extinguishers while simultaneously shutting down the process.  The extinguishers mounted on the dryer/cooler rapidly discharge suppressant in a effort to quench the fireball before maximum pressures are reached.  Isolation extinguishers mounted on the inlet duct and dust extraction ducts mitigate explosion propagation to interconnected process equipment.  An explosion-proof rotary gate valve mounted on the discharge duct reduces the likelihood of burning materials passing downstream.